WO2013/178985, WO2014/170672 and WO2014/170674 disclose known injectable nut caps for forming a sealed cavity around an end of a fastener, the cap comprising: an inner cap terminating at an inner cap rim which surrounds an opening into a central cavity and lies in an inner cap plane around a majority of a circumference of the inner cap; an outer cap fitted over the inner cap, the outer cap comprising an outer cap body and a skirt extending from the outer cap body to a skirt rim; an annular pocket between the skirt and inner cap; and a flow space between the outer cap body and the inner cap. The flow space is in fluid communication with the annular pocket so that it can receive a flow of injected sealing material and feed it into the annular pocket. The sealing material in the annular pocket forms a bead which seals the central cavity and adheres the cap to a structure.
In such injectable nut caps the skirt rim is offset from the inner cap plane so that there is a continuous gap between the skirt rim and the structure. This gap enables air and sealing material to flow out from the annular pocket.
Furthermore, in such injectable nut caps the skirt extends at a small acute angle which increases at the skirt rim to form a raised lip. This raised lip ensures that the outer cap does not clash with features such as raised fillets.
A non-injectable nut cap is described in WO2012/107741. Inner and outer caps are supplied as a kit of parts. A metered quantity of sealant is applied into the outer cap during installation, or could be supplied pre-installed in the outer cap in a frozen state. The outer cap is pushed onto the inner cap, and the sealant is squeezed through a flow space between the outer and inner caps into contact with a structure.